The Japanese Patent No. 2907077 describes an overhead travelling carriage having a centering member provided at the bottom of a platform. The centering member is abutted against the top of an article to be conveyed. This pressing force prevents vibration of the article being conveyed. To ensure that the vibration of the article is prevented, it is possible to increase the pressing force exerted by the centering member on the article. However, the inventor has found that an increase in pressing force on the centering member makes it difficult to grip or release the article using a chuck provided at the bottom of the platform.
FIG. 6 shows problems with the prior art. 8 is a flange of a cassette to be conveyed. 14 is a platform, 16 is a hanging member, and 18 is a chuck. 20 is a centering member and 24 is its shaft. 26 is a plate, and an elastic member 28 such as a coil spring is placed between the centering member 20 and the plate 26 to press the centering member 20 downward. Lowering the platform 14 first brings the centering member 20 into contact with a concave portion 9 of the flange 8. Then, to further lower the platform 14 so that the chuck 18 can support a bottom surface of the flange 8, it is necessary to cancel the weight of the platform 14 using a repulsive force exerted by the elastic member 28 on the platform 14, to further lower the platform 14. However, the platform 14 must be light in weight, so that the pressing force that can be exerted on the centering member 20 is limited. A similar problem occurs when an article is unloaded onto a load port. When the article lands on the load port, the spring force of the elastic member 28 urges the platform 14 upward. Thus, the platform 14 no longer lowers, thus making it difficult to remove the chuck 18 from the bottom surface of the flange 8. Accordingly, an increase in pressing force on the centering member 20 is likely to affect an operation of gripping or releasing the article.